How do i do these questions?
1)How can the word DECISIONS be arranged so that the "N" is somewhere to the RIGHT of "D" (my problem is the two I's in particular)
ANSWER: 45360

2)Bob is about to hang 8 shirts in the wardrobe. He has 4 different styles of shirt, 2 identical ones of each particular style. How many different arrangements are possible if no 2 identical shirts are next to one another?
ANSWER: 864

3) A motorist travels through 8 sets of traffic lights, each of which is red or green. He is forced to stop at 3 sets of lights
a) In how many ways could this happen? ANSWER: 56
b) What other number of red lights would give an identical answer to part a)? ANSWER: 5

4) Find how many 5 digit numbers are possible from the digits 9,8,7,6,5,4,3,2,1,0 if digits are to be in
a) ascending order? ANSWER: 126
b) descending order? ANSWER: 252

Any help on any of these questions really appreciated !!